The IEEE 802.11 wireless LAN standardisation process recently created the “high throughput” task group, which aims to generate a new standard for wireless LAN systems with a measured throughput of greater than 100 Mbit/s. The dominant technology that promises to be able to deliver these increased speeds are so-called MIMO (multiple-input, multiple-output) systems. MIMO systems are defined by having multiple antennae used for both transmission and reception. The maximum theoretical throughput of such a system scales linearly with the number of antennae, which is the reason that the technology is of great interest for high throughput applications. An example of such a system is shown in FIG. 1, with a laptop 2 transmitting to an access point where each device has three antennae.
The reason why these systems can offer improved throughput compared to single antenna systems, is that there is spatial redundancy: each piece of information transmitted from each transmitting antenna travels a different path to each receiving antenna, and experiences distortion with different characteristics (different channel transfer functions). In the example of FIG. 1, there are three different channel transfer functions from each antenna to each receiver: the transfer function from transmitting antenna x to receiving antenna y is denoted by Hxy. Greater capacity is obtained by making use of the spatial redundancy of these independent or semi-independent channels (perhaps in conjunction with other coding techniques) to improve the chance of successfully decoding the transmitted data. The examples given here use three transmitting antennae. However, any arbitrary number of transmit antennae can be used.
There are a wide range of published techniques for encoding information over a MIMO channel set, for example, linear beamforming with a Wiener filter receiver, space time block coding, etc. In virtually all of the techniques, it is necessary to obtain a reasonably accurate estimate of the channel transfer functions at least at the receiver. In some of the techniques, channel transfer function estimates must also be available at the transmitter: it is possible to encode the estimated transfer function at the receiver and send it back to the transmitter if the channel transfer functions change sufficiently slowly with time.
An important criterion of the high-throughput WLAN standardisation activity is that the new systems can interoperate with existing 802.11a and 802.11g OFDM WLAN systems. This means, primarily, that the legacy systems can interpret sufficient information from the transmission of the new system such that they do not interact in a negative manner (e.g., making sure that legacy systems remain silent during an ongoing transmission of the new system). For this reason, it has been proposed that the new high-throughput standard uses the same preamble structure as for 802.11a/g. The preamble is the information transmitted before the data-carrying portion of a transmission, which allows the transmission to be detected and allows estimation of, amongst other things, the channel transfer function. The aim is that the transmitted preambles will be sufficiently similar so that legacy devices can determine the presence and duration of a high-throughput transmission.
A representation of an IEEE 802.11a/g OFDM preamble is shown in FIG. 2. The first portion of the preamble consists of 10 repetitions of a short 0.8 μs long sequence known as the short preamble symbol A. These are used to detect the presence of an incoming signal and to perform initial estimations of, for example, carrier frequency offset.
The second portion B of the preamble uses the same sort of transmission as the OFDM symbols that are used to carry data in the payload of the transmission. The symbols are 3.2 μs long, and are made up of 52 subcarriers with a spacing of 0.3125 MHz, as shown in FIG. 3. The preamble consists of 2 repetitions of a known 3.2 μs training symbol, preceded by a 1.6 μs cyclic prefix (a copy of the last half of a training symbol prepended to the sequence). These OFDM training symbols are used to perform an estimate of the channel transfer function from the transmitting antenna to each receiving antenna. The cyclic prefix CP means that each OFDM subcarrier experiences a flat fading channel (for sufficiently short channel delay spreads). Flat fading means that the channel transfer function for the signal on each subcarrier can be represented purely by a phase rotation and a scaling of amplitude. These amplitude and phase changes for each subcarrier can readily be estimated when the received signal is transformed into the frequency domain (e.g., via the FFT).
The final portion of the preamble, known as the SIGNAL field C, is a single OFDM data symbol (3.2 μs long with a 0.8 μs cyclic prefix) modulated using BPSK, the most robust transmission mode defined in the standard. This contains details of what modulation format is used for the rest of the transmission, as well as the overall length of the transmission.
D represents the data symbols.
There are two primary difficulties in implementing a MIMO system that is interoperable with legacy 11a/11g devices. Firstly, it is necessary to be able to signal that the new MIMO transmission methods are being used while also allowing legacy devices to gather sufficient information of the transmission in progress. This can be done in a straightforward manner: there are unused portions of the 802.11a/11g SIGNAL field, which are defined as reserved (not used in transmission, and ignored on reception). These portions can be used to flag the use of a new transmission mode, while the rate and length information contained in the SIGNAL field can be used to indicate the duration of the transmission. For MIMO devices, this first signal field can then be followed by another signal field, shown in FIG. 4 with the second signal field denoted as SIGNAL2, E.
These portions of the preamble structures in FIGS. 2 and 4, which correspond to each other, have been denoted with the same reference letter.
The legacy device will thereby interpret the SIGNAL C field correctly (ignoring the reserved sections): the remainder of the frame will not be correctly received, but the legacy device will recognise that a transmission is underway and know what the duration of the transmission is. A non-legacy device will interpret both SIGNAL, C and SIGNAL2, E, using the SIGNAL2 field, E to configure the operating mode for the remainder of the transmission (perhaps in conjunction with information from the SIGNAL field C).
A more complicated problem is the task of creating the estimates of the channel transfer function from each transmitting antenna to each receiving antenna. Techniques exists whereby the transfer function at the receiver can be estimated with transmission occurring on all antennae simultaneously; however, these techniques are not compatible with the existing 11a/11g preamble structure. The alternative is that transmissions on each antenna are separated, in time and/or in frequency.
Probably the simplest way to generate channel estimates for each transmit antenna is to separate the transmissions in time. The initial preamble is transmitted on a single antenna. This will allow legacy devices to receive the preamble, and will allow MIMO devices to estimate the channel transfer function from the first transmitting antenna to each receiving antenna. Subsequently, long training symbols can be repeated on each of the other transmit antennae, allowing the channel transfer functions to be estimated from each of the remaining transmit antennae to each receive antenna.
An example of one possible preamble structure using this method is shown in FIG. 5. Here, everything up to the SIGNAL2 field are transmitted on antenna 1, and antennae 2 and 3 then transmit copies of the training sequence (the chosen order of the training sequences and the SIGNAL2 field is unimportant, as long as it is standardised).
An alternative to separating the transmissions in time is to separate the transmissions on each antenna in frequency, so that a given antenna is the only one transmitting on a given subcarrier at a given time, and to use the standard 802.11a/g preamble. An example of a possible distribution is shown in FIG. 6. The subcarrier/Tx antenna distribution can either be used for the whole preamble, or can be used for the long training symbols and the SIGNAL/SIGNAL2 fields only. The channel spacing is 0.3125 MHz.
From the point of view of a legacy device, there is a unique transfer function for each subcarrier that can be estimated that remains constant through to the SIGNAL field and allows the required information to be decoded.
For a MIMO device, the channel transfer functions are not completely known for all subcarriers for each transmitting antenna. It is therefore necessary to exploit the characteristics of the physical channel, whereby nearby subcarriers have a channel transfer function that is correlated with one another. It is therefore possible to make an estimate of the unknown subcarriers interpolated or extrapolated from the nearby subcarriers.
Multiple training symbols give an unambiguous and good-quality estimate for the channel transfer functions. However, they represent a significant overhead (an extra 20 μs per packet). Since the aim of the MIMO system is to provide very greatly increased throughput, this overhead becomes the limiting factor in determining the available transmission rate and fails to meet the required target of 100 Mbps.
The use of the diagonal channel estimate offers a minimal overhead. However, the requirement to interpolate/extrapolate the channel transfer functions causes problems, particularly for difficult channels, due to errors in the resulting estimates. Such channel estimation errors are irreducible (increasing signal power does not improve the situation), and are likely to be a limit to the available data rate. The problem is particularly bad for the subcarriers at the edge of the band, for which extrapolation must be performed (since a known subcarrier channel transfer function exists only on one side).